Golden Rule: Don’t share an AppleID between users. Each user (including minors) should have their own AppleID for their own exclusive use - linked to their own primary email (and secondary/recovery email addresses and trusted telephone numbers). Sharing an AppleID causes multiple practical, data leakage and security issues.
If you haven’t already done so, you should create an AppleID for your child - being careful to set the correct Date Of Birth (this is important). Do not sign-in to Messages, on your child’s device, with your own AppleID.
Consider use of the Apple Family Sharing features. Decide on who will “lead” the family group and become responsible for App and subscription payments. Add other family member’s AppleIDs to the family group - and configure which content, including Apps, that you wish to share and with whom. If so configured, Apps already purchased and installed will remain available to family members.
Family Sharing
Family Sharing - Apple
Set up Family Sharing – Apple Support
Family Sharing and Apple ID for your child - Apple Support
Share App Store, iTunes Store and Apple Books purchases with Family Sharing – Apple Support
Parental Controls
Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support
I hope this brief explanation - and links to relevant resources - prove to be helpful in guiding you to a “secure” and acceptable solution.
